The present disclosure relates to a gas turbine engine combustor and, more particularly, to a “CUNERB” swirler assembly therefor.
Gas turbine engines, such as those which power commercial and military aircraft, include a compressor for pressurizing a supply of air, a combustor for burning a hydrocarbon fuel in the presence of the pressurized air, and a turbine for extracting energy from the resultant combustion gases. The combustor generally includes radially spaced apart inner and outer liners that define an annular combustion chamber therebetween. Arrays of circumferentially distributed combustion air holes penetrate multiple axial locations along each liner to radially admit the pressurized air into the combustion chamber. A plurality of circumferentially distributed fuel injectors axially project into a forward section of the combustion chamber to supply the fuel for mixing with the pressurized air.
Combustion of the hydrocarbon fuel in the presence of pressurized air may produce nitrogen oxide (NOX) emissions that are subjected to excessively stringent controls by regulatory authorities, and thus may be sought to be minimized.
At least one known strategy to minimize NOX emissions is referred to as rich burn, quick quench, lean burn (RQL) combustion. The RQL strategy recognizes that the conditions for NOX formation are most favorable at elevated combustion flame temperatures, such as when a fuel-air ratio is at or near stoichiometric. A combustor configured for RQL combustion includes three serially arranged combustion zones: a rich burn zone at the forward end of the combustor, a quench or dilution zone axially aft of the rich burn zone, and a lean burn zone axially aft of the quench zone.
During engine operations, a portion of the pressurized air discharged from the compressor enters the rich burn zone of the combustion chamber. Concurrently, fuel injectors introduce a stoichiometrically excessive quantity of fuel into the rich burn zone. Although the resulting stoichiometrically fuel rich fuel-air mixture is ignited and burned to release the energy content of the fuel, some NOX formation may still occur.
The fuel rich combustion products then enter the quench zone where jets of pressurized air radially enter through combustion air holes into the quench zone of the combustion chamber. The pressurized air mixes with the combustion products to derich the fuel rich combustion products as they flow axially through the quench zone. The fuel-air ratio of the combustion products thereby changes from fuel rich to stoichiometric which may cause an attendant rise in combustion flame temperature. Since the quantity of NOX produced in a given time interval increases exponentially with flame temperature, quantities of NOX may be produced in this initial quench process. As the quenching continues, the fuel-air ratio of the combustion products changes from stoichiometric to fuel lean which cause an attendant reduction in the flame temperature. However, until the mixture is diluted to a fuel-air ratio substantially lower than stoichiometric, the flame temperature remains high enough to generate NOX.
RQL injector designs operate on the principle of establishing a toroidal vortex followed by vortex break-down and the formation of a re-circulating zone to stabilize the flame and provide continuous ignition to the fresh reactants. This mode of operation results in relatively high shear stresses which, in turn, may lead to pressure oscillations from heterogeneous chemical release rates.
NOx formation is not only a function of temperature, but also of flame residence time and Oxygen concentration in the reaction zone. Increasing the flame strain tends to reduce the residence time in the flame, but may also increase the Oxygen concentration in the flame. These intermediate effects of strain rates tend to increase the production rate of NOx. At high strain rates, however, the reduction in flame temperature overcomes the influence of the Oxygen concentration, and NOx production rates are reduced.
Dry Low NOx (DLN) combustors utilize fuel-to-air lean premix strategy which operate near flame stability envelope limits where noise, flame blow-off (BO), and flashback may affect engine performance. For this reason, DLN strategy is limited to land-based Ground Turbine applications.